Editor's note: Julian Zelizer is a professor of history and public affairs at Princeton University. He is the author of "Jimmy Carter" and of the new book "Governing America."
(CNN) – Now that the conventions are over, CNN Contributor Julian Zelizer says the final phase of campaign begins. Given how close this race is, and the handful of states that will determine the outcome, each of these three challenges has the capacity to produce a significant swing in November:
1. The advertising wars:
[0:33] "Whether voters like it or not, the truth is that much of the fall revolves around character assassination on television and the radio, and now the Internet."
2. The presidential debates:
[1:30] "It will be a high visibility moment when both candidates can shine or stumble as a result of their physical actions and rhetorical skills."
3. The "ground game":
[2:19] "Even in an age of the Internet and smart phones, have dedicated people working to get out the vote in communities throughout the country can have a huge impact."